Issue 43

A. Luciani et alii, Frattura ed Integrità Strutturale, 43 (2018) 241-250; DOI: 10.3221/IGF-ESIS.43.19 242 The behavior and effectiveness of rockfall protection net fences was tackled both by full scale tests [9-13], especially after the development in Europe of the ETAG 027 standards [14-16], by pseudo-static analyses [17] and by numerical modelling [18-28] but a complete analysis of aged rockfall net fences has not been performed. For this reasons, in this paper the influence of damages and installation problems on the efficiency of rockfall protection net fences has been studied by using numerical modelling. The assessment of the possible damages and installation problems has been performed based on an analysis of data obtained during a site survey in the Alps reported by Dimasi et al. [29]. The numerical simulation was therefore developed with the simulation of a standard impact, according to ETAG027, with different conditions able to simulate different degree of ageing or damages on the various elements of the structure. These models allow to determinate the possible reduction of efficiency and to compare the behavior of the deteriorated and not deteriorated rockfall protection net fences. The simulation is performed with a FEM model of a rockfall protection net fence of nominal energy of 3000 kJ. The model is assessed using the results of experiments on full-scale prototypes whose data were obtained from Gottardi and Govoni [11]. N UMERICAL MODEL OF THE NET FENCE he numerical model was developed using the software ABAQUS/Explicit 6.13. This software has an explicit finite element formulation allowing to simulate non-linear dynamic events such as the impact of a block on a net fence. The studied net fence is a commercial product with a Maximum Energy Level (MEL) of 3000 kJ and the full scale data reported by Gottardi and Govoni [11] were used for back analysis. The support structure of this net fence has four HEA200 steel posts, 5 m high, restrained at the base by cylindrical hinges allowing rotation in the upstream-downstream direction. The interception structure is made of a principal steel ring net; each 350 mm ring is connected to six nearest rings. The connection structure comprises two longitudinal upper cables and two longitudinal lower cables, eight upstream cables and four lateral cables. Each cable has a diameter of 20 mm. The longitudinal cables are free to slide on the posts in the longitudinal direction. The net fence is provided with tubular energy dissipating devices, one on each upstream cable and three on each longitudinal cable. Fig. 1 shows a photo of the net fence. Figure 1: Photo of the studied rockfall protection net fence. In the numerical simulation, the support structure was modelled with 3D-2node beam elements, with a HEA200 cross section. These elements had linear elastic behavior with Young’s modulus of 210 GPa while the cables, the energy dissipating devices and the net were modelled with 3D-2node truss elements that cannot withstand flexural stresses. The material assigned to the cables had elasto-plastic behavior with Young’s modulus of 150 GPa in the elastic part of the curve and of 5 GPa in the plastic one. The yield strain was set at 0.001 and the ultimate strain at 0.006, which corresponds to an ultimate stress of 1770 MPa. T